A big part of a Level Designer's job is to keep building and tweaking an area to make it as fun as possible. This often means we are pushing further than frame rate will allow and at some point in production we usually push too far. At this point on EVERY project I've worked on, there has been a promise from the tech groups that optimizations are coming down the road that will make all of what we're doing just fine.
While it has always been true that optimizations were on the way, it seems that people always expect much more from this pass than is ever able to be produced. I remember on The Simpsons Game when I was working on the Springfield Hub and the frame rate was running at about 12fps when running in split-screen Co-Op. Because of the vast real estate needed to build Springfield and some constraints in layout I had from one of the missions taking place there, I was kind of restricted in what I could do on my end to improve it. Luckily we didn't have our LOD system in place yet and was ensured that once it was working things would be fine.
Well, the LOD system came and while it did help quite a bit...we were still way under acceptable frame rate. So, I spent a ton of time working with the outsourced art team I was managing to see where we can reduce poly counts, etc. In the end, we couldn't do enough and it was so late in the project that the decision was made to cut Co-Op in the Hub. This now meant that players would play a mission, then the 2nd controller would be unusable while the 1st player navigated to the next mission. This also meant the 300 collectibles and extra fun stuff I had spent months building into the hub could never be enjoyed by a 2nd player and would probably be ignored by the 1st player in order to get to more Co-Op gameplay.
Unfortunately, I can probably come up with a similar story for every game that I've worked on. I've now learned to not every rely on major help from an optimization pass and build everything as close to functional as possible from the start. If I then get some extra room to work with down the road, I can use it to add back in extra awesome!